The invention relates to Local Area Networks (LANs) and to bridges-and routers that are used on such networks.
Bridges are devices that connect local area networks (LANs) together to form what are referred to as Extended LANs. Large Extended LANs have proven to be difficult to manage because of fault-isolation and addressing problems. The present invention enables a LAN manager to divide a large Extended LAN into smaller virtual LANs that have less overhead and are easier to manage. It further allows the LAN manager to interconnect the virtual LANs with a router.
The recent emergence of large multiport bridges, such as GIGAswitch from Digital Equipment Corporation, which can bridge up to 22 FDDI LANs, enable users to create a large extended LAN. That is, logically it appears that all stations that are bridged together by the switch are on a single LAN. This large configuration is reasonable if the bridge is at the periphery of the extended network and is responsible for bridging together a small number (say 100-250) of stations. However, there are two disadvantages if the bridge is used as the backbone of a large extended LAN. First, implementation and addressing limitations may limit the number of stations that can be present on a single Extended LAN. For example, it is well-known that broadcast traffic used in a LAN does not scale well as the number of LAN stations increases. The second problem is the lack of "firewalls" between the individual LANs that are bridged together by the bridge. An error on one LAN caused by a particular protocol failure can cause all other protocols on the LAN to fail. For example, if a set of stations on a particular LAN get stuck in a loop where they keep generating broadcast traffic, then the entire Extended LAN can fail. Thus some users choose to use a device called a router (as opposed to a bridge) to interconnect LANs.
There are several well-known differences between bridges and routers which make interconnecting LANs with routers more flexible and easier to manage. Routers allow users to construct extremely large and yet manageable networks. Some reasons for this are as follows. First, routers typically do not allow broadcast traffic; if they do, the broadcast traffic can be carefully controlled. By contrast, bridges must allow broadcast to allow LAN protocols to work correctly. Second, routers can be used to break up networks into a hierarchy of manageable subnetworks; bridges cannot. Third, routers have access to more information fields in messages than do bridges; this allows routers to have more discrimination in enforcing security and performance policies.